The present invention relates to an optical signal detector for detecting signal light superposed on a part of input light which contains background light. More particularly, the present invention relates to an optical signal detector with which an optical signal superposed on intense background light can be observed at high time resolution.
The transient behaviors of ultra-fast optical phenomena can be measured by a variety of means. One method is the use of a streak camera which is operated by the following principle: incident light is focused on a photocathode where the photons are converted to electrons; the photoelectron beam emitted from the photocathode is then swept at high speed by applying a deflection voltage; and thereby the temporal change in the intensity of the incident light is measured as the change in brightness associated with position on a phosphor screen.
The heart of the streak camera is a streak tube which is generally indicated as 13 in FIG. 19 and which includes the following main components: a photocathode 14 where an optical image (slit image) focused at an entrance window through a slit plate 10 and a lens 12 in an input optical system is converted to an electron image; an accelerating electrode 16, typically in mesh form, which accelerates the electron image produced on the photocathode 14; (main) deflecting electrodes 22 by which the accelerated photoelectrons are swept at high speed in a direction perpendicular to the slit length direction (either upward or downward as viewed in the drawing); and a phosphor screen 26 on which the deflected photoelectron image is converted to an optical image (called "streak image" which carries brightness information with the lapse of time being expressed by position on the vertical axis), which emerges from an exit window.
The other components shown in FIG. 19 are as follows: a focusing electrode 18 by which the photoelectrons accelerated by the electrode 16 are converged to have a specified cross-sectional area; an anode 20 having an opening area in the center; a (main) sweep voltage generator circuit 23 which applies a predetermined (main) sweep voltage to the deflecting electrodes 22 in synchronism with the passage of electrons; a microchannel plate (MCP) 24 by which the electrons passing through the deflecting electrodes 22 are multiplied before they arrive at the phosphor screen 26; a cone-shaped shielding electrode 25 that is provided on the input side of MCP 24 and which improves the precision of measurements by blocking the electrons deflected to go outside the effective sweep range of phosphor screen 26; and an image pickup device 28 such as a high-sensitivity TV camera, e.g., a silicon intensified target (SIT) camera or a CCD camera, which picks up the streak image via a lens 27 in an output optical system.
Operating by the principles described above, streak cameras are roughly divided into two types according to the method of sweeping; namely, a single scan type and a synchroscan type. A single scan streak camera performs linear sweep with an ultra-fast sawtoothed wave repetitive at a rate not higher than several kilohertz in synchronism with pulse laser light. A synchroscan streak camera performs high-speed repetitive sweep with a sinusoidal wave synchronized with laser light pulses repetitive at 80-160 MHz. An improved version called a synchronous blanking streak camera has also been developed. As shown in FIG. 20, this camera has sub-deflecting electrodes 29 crossed with the main deflecting electrode 22 and performs elliptical sweep in such a way that retrace sweep is laterally shifted to avoid scanning on the phosphor screen 26, thereby insuring that a signal associated with main sweep is selectively measured in a correct way.
These prior art streak cameras are described in many patent documents such as Japanese Patent Nos. 1,149,098, 1,149,120 and 1,099,753, JP-A-59-58745 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-61-183857, U.S. Pat. Nos. 4,232,333, 4,352,127, 4,611,920 and 4,661,694, British Patent Nos. 2,042,163, 2,044,588 and 2,131,165.
The use of streak cameras in measuring the transient behaviors of ultra-fast optical phenomena has the following advantages: first, it provides a purely electronic direct method having fast time resolution and high detection sensitivity; second it is capable of measuring single (non-repetitive) phenomena; third, the streak image which is inherently a two-dimensional image helps provide two-dimensional measurements such as time-resolved spectral measurements and space- and time-resolved measurements, as well as multi-channel measurements; and fourth, by properly selecting the materials of which the photocathode and entrance window are made, measurements over a broad spectral range extending from the near-infrared region through the vacuum ultraviolet region up to the X-ray region can be realized.
A sampling optical oscilloscope has also been commercialized. As shown in FIG. 21, the streak image is electronically sampled with a sampling streak camera 30 in which a slit plate 32 having an electronic sampling slit 32A that limits said streak image spatially is provided typically in the streak tube. FIG. 21 also shows a photodetector 34 that detects the light emission intensity caused by electrons impinging on the phosphor screen 26 and may be composed of a photomultiplier tube, a high-sensitivity photodiode, an avalanche photodiode, a PIN photodiode, etc. This sampling optical oscilloscope is described in such patent documents as JP-A-59-104519, JP-A-59-134538, JP-A-59-135330, U.S. Pat. Nos. 4,645,918 and 4,694,154, and British Patent No. 2,133,875.
However, all of the prior art streak cameras described above perform immediate photoelectric conversion on input light and it has been difficult to observe with these cameras the waveform of signal light superposed on intense background light (dc light).